1. Field of the Invention
This invention relates to a liquid crystal display device, and more particularly to a ferroelectric liquid crystal composition and a ferroelectric liquid crystal display device.
2. Description of the Prior Art
Recently liquid crystal displays are used not only in watches and pocket calculators but also in a wide range of image display equipment including color televisions. Nematic liquid crystal is mainly used in the liquid crystal color displays. The nematic liquid crystal, however, has characteristics which are far from the ideal, and has a number of problems. The ferroelectric liquid crystal has characteristics such as a speedy response and a memory capability which are not possessed by the nematic liquid crystal, and has been studied in many aspects for use in display devices (Optronics, 1983, No. 9), while a practical use has not been realized yet. One of the reasons is that the bistability and the threshold characteristic deteriorate as time passes, causing a problem that superior initial characteristics can not be maintained.
In FIG. 3, a time-dependent deterioration of the threshold characteristic is shown. The cell gap is 2 .mu.m. A mark O shows the maximum relative brightness when a voltage is applied, which is a bulk response, and a mark X shows a relative brightness after scanning 1,000 lines of a series of measurement waveforms, which is a memory response. FIG. 3(a)1 and 3(a)2 shows a threshold characteristic immediately after the injection of liquid crystal, in which an acute threshold characteristic and a superior bistability are shown. In FIG. 3(b)1 and 3(b)2 is shown a result of experimentally studying a time-dependent deterioration of a liquid crystal panel having such a superior initial characteristic. In FIG. 3(b)1 and 3(b)2, which shows a threshold characteristic after 200 hours have passed, thresholds become asymmetric in comparison with FIG. 3(a)1 and 3(a)2, and as longer time passes, it was recognized that the asymmetricity tended to grow to be monostable. Therefore, for a practical use, a long-term stability of the characteristic is an essential subject.